Vehicle control systems may be configured to start an engine assuming a given intake manifold volume. However, interactions between vacuum levels in a brake booster and the intake manifold pressure at engine starts can cause variability in the air charge, and consequently air-to-fuel ratio at the engine starts. As such, this increases exhaust emissions.
One approach to address this variability is shown by Kayama et al. in U.S. Pat. No. 6,857,415. Therein, a valve is placed between the brake booster and the intake manifold to equalize the (remaining) pressure in the brake booster to atmospheric levels or to remove air from the intake manifold to the brake booster.
However, the inventors herein have identified a potential issue with such an approach. As one example, the valve used in the approach of Kayama et al. does not allow the level of intake manifold pressure (MAP) to be set from one engine start to another engine start. As another example, even with the valve, a consistent MAP level may not be attained at engine starts occurring at high altitudes as well as at sea level.
In one example, some of the above issues may be at least partly addressed by a method for starting an engine comprising positioning a throttle based on a vacuum reservoir pressure during a start. For example, in one embodiment, the throttle may be positioned based on an initial pressure in a brake booster during an engine start. By adjusting the position of the throttle at the engine restart, the rate at which air enters the engine may be controlled to be more consistent. Additionally, since manifold pressure at initial engine fueling affects both cylinder air charge and fuel vaporization, both consistency and accurate control can be used to improve air-fuel control. In this way, better air-to-fuel ratio control can be achieved during an engine start, thereby reducing emissions and improving the quality of the environment.
Note that in one example, the throttle positioning can include first fully closing the throttle, and then opening it to a position that is based on the reservoir pressure level identified before the engine start (e.g., before engine cranking while the engine was at rest). Alternatively, or additionally, the pressure level may be monitored during the cranking and run-up to identify the setting of the throttle.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.